The present invention relates to a system and method for automatically running a vehicle at a cruising speed.
Japanese Patent Application First Publications Sho Nos. 47-35692, Sho 56-99518, Sho 60-71341, and Sho 60-50031 and Japanese Patent Application Second Publication Sho No. 53-7592 exemplify vehicle cruising speed running systems.
In systems as disclosed in the above-identified Japanese Patent Application publications, a set switch is provided for setting a vehicle speed to a desired cruising speed when turned on. When the set switch is operated, a vehicle speed adjusting mechanism associated with a throttle valve of an engine mounted in a vehicle is actuated to adjust an angular displacement of the throttle valve so that the vehicle speed matches with a set cruising speed.
In addition, when a brake pedal or clutch pedal is operated, a control to maintain the vehicle speed at the set cruising speed is released and the vehicle speed is thereafter reduced. On the other hand, a system as described above includes a resume switch. When the resume switch is operated, the system automatically returns the vehicle speed to the set cruising speed before the operation of such a pedal as described above (resume function) at a constant acceleration. Furthermore, an acceleration switch is provided for setting the vehicle speed at a constant acceleration when turned on. When the acceleration switch is turned on, the constant acceleration of the vehicle is started in which the opening angle of the throttle valve is adjusted so that the actual acceleration of the vehicle accords with the set acceleration. Thereafter, when the acceleration switch is turned off, the vehicle cruising speed control is started in which the vehicle speed is adjusted to reach a target vehicle speed which is the vehicle speed at the time of an off operation of the acceleration switch. When the actual acceleration exceeds the set acceleration during the transfer of control from the constant acceleration to the cruising speed, a gain of vehicle speed control is corrected so as to prevent an overshoot of the vehicle speed.
Among these Japanese Patent Application Publications, for example, in the Japanese Patent Application First Publication Sho No. 47-35692, the vehicle cruising speed running system calculates an error .epsilon..sub.I (=V.sub.s -V) between the vehicle speed V and a target vehicle speed V.sub.s for each predetermined period (, e.g., 300 msec.) and an error .epsilon..sub.p (=V(t-.DELTA.t)-V(t)) between the present vehicle speed V(t) and a previous vehicle speed V(t-.DELTA.t) before the predetermined period .DELTA.t. In addition, a change rate .DELTA..theta. of an opening angle through which the throttle valve is angularly displaced is calculated as follows: EQU .DELTA..theta.=K.sub.I .multidot..epsilon..sub.I +K.sub.p .multidot..epsilon..sub.p ( 1)
However, in systems disclosed in the above-identified Japanese Patent Application publications, the change rate .DELTA..theta. described above is neither rested on a change in a running resistance of a vehicle during the vehicle run on a road having a gradient nor a vehicle weight. In addition, the system merely operates to displace the throttle valve in an open direction by .DELTA..theta. for each predetermined period of time when the change rate .DELTA..theta. derived from the above equation (1) indicates positive and operates to displace the throttle valve in a close direction by .DELTA..theta. for each predetermined period of time when the change rate .DELTA..theta. indicates negative. Therefore, since the system cannot determine each predetermined period at which vehicle speed a value of the opening angle of the throttle valve thus determined is settled, the opening angle of the throttle valve is unnecessarily varied when the vehicle runs on a rugged road. Consequently, the vehicle driver feels an ride.
To eliminate such an unpleasant feeling, the Applicant has proposed such a vehicle cruising running system in which a running resistance of the vehicle is estimated and the throttle valve is feedforward controlled on the basis of the estimated running resistance.
An algorithm which estimates the running resistance and proposed by the Applicant is as follows.
STEP 1: A throttle opening angle .theta. and engine torque T.sub.e corresponding to the engine speed are read from an engine performance curve.
STEP 2: A wheel force F.sub.W is calculated from the following equation. EQU F.sub.W =.gamma.m.multidot..eta.m.multidot..gamma.n.multidot..eta.n.multidot.T.sub .e /R (2)
STEP 3: An acceleration .alpha. of the vehicle is calculated from the following equation. EQU .alpha.={V(t-.DELTA.t)-V(t)}/.DELTA.t (3)
STEP 4: The estimated running resistance D is calculated from the equations (1) and (2) as follows:
D=F.sub.W -Wx.alpha./g (4)
In the equations (2) and (3), .gamma.m: gear ratio of each stage of a transmission, .eta.m: transmission efficiency of each stage of the transmission, .gamma.n: final reduction gear ratio, .eta.n: final transmission efficiency, R: radius of tire, W: vehicle weight, and g: gravitational acceleration.
However, since a dead time and lag element are generally present in a transient response of a wheel force F.sub.W with respect to a stepwise angular change of the throttle valve, an error occurs in the calculation of the wheel force F.sub.W this error is due to the presence of the dead time and lag elements in the situation where the opening angle .theta. of the throttle valve needs to be largely changed for each control period when the vehicle runs on a rugged road. Consequently, a value of the estimated running resistance D calculated in accordance with the equation (3) is diverged from a true value, therefore a target opening angle of the throttle valve is erroneous, and thus the opening angle value of the throttle valve cannot properly be controlled.